Typically, a phase-change optical recording medium (phase-change optical disc) such as DVD+RW has a basic structure in which a recording layer composed of a phase-change material is formed on a plastic substrate, and a reflective layer that allows for improving optical absorptance of the recording layer and has a thermal diffusion effect is formed on the recording layer, and such a typical phase-change optical recording medium is irradiated with a laser beam from the substrate surface side, thereby information can be record and reproduced. A phase-change recording material is phase-changed between an amorphous phase and a crystalline phase by application of heat of laser beam irradiation and subsequent cooling. When, after a rapid heating treatment of a phase-change recording material, the phase-change recording material is immediately quenched, the phase-change recording material is changed into an amorphous phase, and when the phase-change recording material is slowly cooled after the rapid heating treatment, it is crystallized. These characteristics are applied for recording and reproducing information in phase-change optical recording media.
Further, for the purpose of preventing oxidation, transpiration or deformation of a recording layer which are caused by application of heat for light beam irradiation, a phase-change optical recording medium is typically provided with an under protective layer (also called as an under dielectric layer) in between a substrate and a recording layer, and an upper protective layer (also called as an upper dielectric layer) in between the recording layer and a reflective layer. These protective layers can respectively function to control optical properties of the optical recording medium by adjusting the thickness thereof, and the under protective layer also has a function to prevent the substrate from softening due to heat applied to the recording layer during recording.
In recent years, with increased information volume handled with computers, recording capacities of rewritable optical discs such as DVD-RAM, DVD-RW and DVD+RW are largely increased, and high-density recording of information is advanced. DVD has a recording capacity of around 4.7 GB, however, in the days to come, requests for high-density recording are further expected to increase. Along with increased information volume, it should be considered that enhancement of recording speed is also required. Presently, as a rewritable DVD disc, an optical recording medium with a single-recording layer that allows for 8× recording speed is commercially available. As a method for allowing for high-density recording using such a phase-change optical recording medium, for example, there have been proposals to shift a wavelength of a laser beam to be used toward shorter wavelengths to blue region or to enlarge the numerical aperture (NA) of an objective lens used for optical pick-up to record or reproduce information to thereby change the spot size of the laser beam applied to an optical recording medium to smaller, and these methods have been studied, developed and put into practical use.
For a technique to increase recording capacity by improving an optical recording medium itself, various types of two-layered phase-change optical recording media are proposed that are produced such that two information layers each composed of at least a recording layer and a reflective layer are laid one on top of another on one surface of a substrate, and the two information layers are bonded together with an ultraviolet curable resin or the like. A separation layer (called an intermediate layer in the present invention) which is the bonded part between these information layers has a function to optically separate the two information layers and is composed of a material that less absorbs laser beam or avoid absorbing laser beam unnecessarily because the laser beam used for recording and reproducing needs to reach the innermost information layer as viewed from the laser irradiation side.
There are still many problems existing in such a two-layered phase-change optical recording medium.
For example, when an information layer (a first information layer) disposed at the front side as viewed from the laser beam irradiation side is not sufficiently transmissive to a laser beam, it is impossible to record and reproduce information on a recording layer of another information layer (a second information layer) disposed at the innermost side as viewed from the laser beam irradiation side, and thus a reflective layer constituting the first information layer must be a ultrathin semi-transparent reflective layer. As the result, it is difficult to obtain light transmittance and heat dissipation effect, and it is necessary to form a heat diffusion layer (a light transmissive layer) so as to make contact with the semi-transparent reflective layer to increase the light transmittance and supplement the heat dissipation effect. Further, as compared to conventional single-layer phase-change optical recording media or conventional two-layered recordable optical recording media, the reflectance of a two-layered phase-change optical recording medium is extremely low of about one third of that of each of these conventional media. Thus, it is considered that it may be difficult to record and reproduce information on the first information layer as well as on the second information layer even with stable tracking ability provided. Further, since the first information layer has a layer structure in which heat is hardly dissipated, from the perspective of storage stability, it is not conceivable that any materials may be used for the recording layer just because a phase-change material is used. Actually, it is necessary to limit materials and the composition of the recording layer for the first information layer.
As a known technique, Patent Literature 1 discloses a two-layered phase-change optical recording medium provided with a substrate having a groove depth of 10 nm to 30 nm and using a laser beam having a wavelength of 360 nm to 420 nm, however, an upper protective layer in a first recording layer structure of the two-layered phase-change optical recording medium is composed of ZnS—SiO2, and the layer composition differs from that of the multi-layered phase-change optical recording medium of the present invention.
Patent Literature 2 discloses an optical recording medium provided with a thermal diffusion layer containing an In oxide and an Sn oxide as the main components, in which the content of the In oxide is 90 mol % or more. Patent Literature 3 discloses an optical recording medium provided with a thermal diffusion layer containing an Sn oxide and an Sb oxide, and Patent Literature 4 discloses an optical recording medium provided with a thermal diffusion layer containing an In oxide and a Zn oxide as the main components, in which the content of the In oxide is 50% or more. However, these disclosed optical recording layer respectively have a different layer composition from that of the optical recording medium of the present invention, and there is no description on groove depth of the substrate, unlike the present invention.
Patent Literature 5 discloses a two-layered recordable optical recording medium with recording layers composed of an organic dye or organic dyes, in which the range of the groove depth is 4λ/16n to 7λ/16n, however, the two-layered recordable optical recording medium can ensure a reflectance of about 20%, and the optical recording medium configuration basically differs from that of a two-layered phase-change optical recording medium, and the groove depth range also differs from that of a two-layered phase-change optical recording medium.
Further, Patent Literature 6 describes a two-layered optical recording medium in its claim 8, however, the optical recording medium configuration is basically different from that of the two-layered phase-change optical recording medium of the present invention.
Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 2004-005920
Patent Literature 2 Japanese Patent Application Laid-Open (JP-A) No. 2004-047034
Patent Literature 3 Japanese Patent Application Laid-Open (JP-A) No. 2004-047038
Patent Literature 4 Japanese Patent Application Laid-Open (JP-A) No. 2005-004943
Patent Literature 5 Japanese Patent Application Laid-Open (JP-A) No. 2005-004944
Patent Literature 6 International Publication No. WO02/029787